If you go to
http://vislab.bjmu.edu.cn/apps/koreader/ota/ you should also see .targz files it is that that you would download, selecting the one for your device. (For now though you might as well download from Github, as only the current stable is being offered.) The "tar" in .targz needs to be removed to have a normal .gz compressed file of the whole update. The same for
here.
Luck;
Ken